1. Field of the Invention
The present invention relates to a projection-type display device which can be applied to for example a projector device for projecting an optical image spatially modulated by reflection-type liquid crystal panels onto a screen.
2. Description of the Related Art
In the related art, a projection-type display device has been proposed which is configured to use reflection-type liquid crystal panels to generate spatially modulated optical images and to project the optical images onto a screen so as to form a desired color image.
Among such projection-type display devices, ones have been proposed which use dichroic mirrors or use dichroic prisms as the means for breaking down illumination light obtained from a light source into red, blue, and green illumination light for supply to corresponding reflection-type liquid crystal panels and for synthesizing the red, green, and blue optical images obtained from the reflection-type liquid crystal panels.
FIG. 1 is a view of the configuration of a projection-type display device using dichroic prisms.
In this projection-type display device 1 using dichroic prisms, as shown in FIG. 1, a light source 2 is comprised for example a discharge lamp 3 and a reflector 4 and emits white illumination light.
Further, the light source 2 uses fly eye lenses 5A and 5B to make the distribution of the amount of the illumination light uniform, then emits the light to a plane polarization conversion element 6. Here, the plane polarization conversion element 6 selectively transmits mainly the s-polarization component and converts the p-polarization component orthogonal to this to the s-polarization component.
Due to this, the light source 2 emits illumination light increased in the polarization component effective for the image display in the illumination light projected from the discharge lamp 3 by the various plane polarizations and reduced in the polarization component orthogonal to this. As a result, the efficiency of utilization of the illumination light is improved by that extent and the contrast of the display image is improved.
A convex lens 7 converges and emits this illumination light on the path of the illumination light emitted from the plane polarization conversion element 6.
A cold mirror 8 emits the components of the illumination light emitted from the convex lens 7 other than the infrared region reflected in a direction 90 degrees from the path of incidence.
A convex lens 9 converges and emits the illumination light reflected at the cold mirror 8.
A polarization beam splitter 11 is formed by adhering inclined planes of rectangular prisms to each other and is formed with a detecting plane 11A at the adhered planes. The polarization beam splitter 11 selectively reflects and emits from the detecting plane 11A the illumination light due to the s-polarized light emitted from the convex lens 28, while selectively transmits the p-polarization component in the synthesized optical image incident on it traveling back along the path of the illumination light due to the s-polarized light and returns the s-polarization component to the light source 2.
A dichroic prism 12 is formed by adhering inclined planes of three prisms each having a predetermined form to each other and is arranged so that the adhered planes out across the path of the light projected from the polarization beam splitter 11. The dichroic prism 12 is formed with dichroic films MB, MR obtained by lamination of dielectric films to a predetermined thickness on the adhered planes cutting across the optical path. The blue and red illumination light in the illumination light projected from the polarization beam splitter 11 are successively selectively reflected at the dichroic films MB, MR. Due to this, the dichroic prism 12 breaks down the illumination light projected from the polarization beam splitter 11 into blue, red, and green illumination light and supplies them to the blue, red, and green color reflection-type liquid crystal panels 13B, 13R, and 13G arranged at the bottom surface of the prism.
The reflection-type liquid crystal panels 13B, 13R, and 13G are driven by corresponding color signals. The illumination light incident by the s-polarized light is reflected with the plane polarization rotated for every pixel. Due to this, optical images changed in plane polarization in accordance with the color signals are projected.
The dichroic prism 12, conversely to the case of the illumination light, synthesizes the blue, red, and green optical images obtained from the reflection-type liquid crystal panels to generate a synthesized optical image and projects the synthesized optical image to the polarization beam splitter 11.
Specifically, the synthesized optical image travels back along the path of the illumination light due to the synthesized light of the p-polarized light and s-polarized light in accordance with the color signals and is emitted to the polarization beam splitter 11. Further, only the p-polarization component in the synthesized optical image passes through the polarization beam splitter 11 and is projected to the projection lens 14.
In this way, the projection lens 14 projects the synthesized optical image passing through the polarization beam splitter 11 to the screen 15. Due to this, a color image is displayed by enlarging and projecting onto the screen the images generated by the reflection-type liquid crystal panels 13B, 13R, and 13G.
Further, a projection-type display device using diehroic mirrors is configured to break down the illumination light incident from a polarization beam splitter into red, blue, and green illumination light by dichroic mirrors instead of the dichroic prisms 12 and project them onto the reflection-type liquid crystal panels and to synthesize the optical images projected from the reflection-type liquid crystal panels and emit the result on a projection lens.
In this type of projection-type display device 1, however, there has been the disadvantage that the so-called haze phenomenon occurs where light is also projected at portions which should inherently be displayed black and those portions are displayed whitish and this haze phenomenon causes a reduction in the contrast of the projected image.
The haze phenomenon will be explained in further detail below.
In the projection-type display device 1, the portion which inherently should be displayed black is reflected without any rotation of the plane polarization of the corresponding illumination light at the reflection-type liquid crystal panels. As a result, in the projection-type display device 1, the corresponding optical images are returned to the light source 2 side by the polarization beam splitter 11. Due to this, the corresponding portion should be displayed black on the screen 15.
In the projection-type display device 1, however, this type of optical image which should be detected at the polarization beam splitter 11 by the s-polarized light is detected by the synthesized light of the s-polarized light and the p-polarized light. Due to this, this type of haze phenomenon is generated.
That is, in an optical system provided with a polarization beam splitter 11, dichroic prisms 12, etc., a phase difference is given in the direction of vibration of the light to the p-polarized light parallel to the boundary planes and the s-polarized light orthogonal to the p-polarization component using as a reference the incidence plane and emission plane of the polarization beam splitter 11, the light detecting plane, the boundary plane of the dichroic film etc. Due to this, in this type of projection-type display device, when viewed as an optical system as a whole, the direction of the p-polarization component initially separated by the polarization beam splitter 11 changes at the boundary planes. Further, the phase difference generated at the boundary planes in this way changes by the incident wavelength and angle of incidence to the boundary planes.
As a result, in the projection-type display device 1, the states of polarization change in the illumination light and optical images propagated through the optical system. Due to this, light is mixed into the portions inherently to be displayed black by the s-polarized light and the haze phenomenon occurs.
FIG. 2 is a view for explaining the changes in the states of polarization. FIG. 2 corresponds to the configuration of the above-mentioned FIG. 1 and shows the case where the illumination light incident from the convex lens 9 is reflected at the polarization beam splitter 11, then successively passes through the dichroic films MB, MR, and strikes the reflection-type liquid crystal panel 13G where it is reflected without modulation. Note that below the letter B will be added to the references to indicate a vector.
In this case, assume that the unit vector showing the direction of the incident illumination light is the direction cosine BC0 and the direction cosines showing the directions of the illumination light at the boundary planes of the detecting plane 11A of the polarization beam splitter 11 and the dichroic film MB and dichroic film MR, all boundary planes, are BC1, BC2, and BC3. Further, the direction cosines showing the directions of the optical images at the corresponding boundary planes after reflection by the reflection-type liquid crystal panels 13G are BC4, BC5, and BC6. Further, the unit vectors showing the arrangement of the boundary planes corresponding to these direction cosines are made normal vectors and indicated by the references BD1, BD2, BD3, BD4, BD5, and BD6.
The s-polarization components BESn orthogonal to the incidence planes of the boundary planes are defined by the following equation with the direction of advance defined by the outer product of the direction cosines and the normal vectors: EQU BESn=BCn.times.BDn/.vertline.BCn.times.BDn.vertline. (1)
(where, n=1 to 6) PA1 (where, n=1 to 6)
Further, the direction cosines of the p-polarization component parallel to the incidence planes of the boundary planes intersect the direction of advance of the s-polarization components BESn at right angles and are expressed by the vector products of the following equation: EQU BEPn=BESn.times.BCn/.vertline.IBESn.times.BCn.vertline. (2)
At this time, the direction cosines become BC2=BC3.noteq.BC1, BC4=BC5.noteq.BC6. Due to the refraction at the polarization beam splitter 11, only the direction cosines BC1 and BC6 differ. Note that the normal vectors are BD1.apprxeq.BD2, inner product BD2.multidot.BD3.noteq.0, BD5.apprxeq.BD6, inner product BD4.multidot.BD5.apprxeq.0.
It is possible to obtain the relationship of the following formula from formula (1), formula (2), and the relationship of the direction cosine BCn and the normal vector BDn. Note that the orthogonal p-polarization component BEPn becomes the same relationship. EQU BES1.apprxeq.BEs2.noteq.BES3 (3)
FIGS. 3A to 3J are views of the states of polarization around the boundary planes by the absolute coordinate system x-y seen from the reflection-type liquid crystal panel side.
As shown in FIG. 3A, at the reflection side of the detecting plane 11A of the polarization beam splitter 11, the illumination light due to the direction cosine BC0 strikes the polarization beam splitter 11. Only the s-polarization component is selectively reflected in accordance with the direction of the p-polarization component and the s-polarization component determined at the detecting plane 11A and becomes linear polarized light.
As opposed to this, in front of the boundary plane of the dichroic film MB, as shown in FIG. 3B, the directions of the p-polarization component and s-polarization component differ slightly from the time of reflection at the polarization beam splitter 11 (the p-polarization component and s-polarization component at the dichroic film shown by the broken line rectangle). Due to this, the illumination light is broken down into the p-polarization component and the s-polarization component at the dichroic film MB and given a phase difference (BES1.apprxeq.BES2).
As a result, after the boundary plane of the dichroic film MB, as shown in FIG. 3C, the illumination light becomes elliptical polarized light.
Further, in front of the boundary plane of the dichroic film MR, as shown in FIG. 3D, the directions of the p-polarization component and s-polarization component largely differ. The illumination light is broken down at the dichroic film MR into the p-polarization component and s-polarization component which are then given a phase difference (BES2.noteq.BES3).
As a result, as shown in FIG. 3E, after the boundary plane of the dichroic film MR, the illumination light can become elliptical polarized light with a largely increased short diameter. When reflected without any polarization at the reflection-type liquid crystal panel 13G, the reflected light becomes elliptical polarized light as showing the front of the boundary plane of the dichroic film MR in FIG. 3F.
The optical image projected from the reflection-type liquid crystal panel as the elliptical polarized light in this way, as shown in FIGS. 3F to 3I, in the same way as the illumination light, is successively broken down into the corresponding p-polarization component and s-polarization component by the dichroic films MR, MB. As shown in FIG. 3J, when striking the detecting plane 11A of the polarization beam splitter 11, an s-polarization component is generated with respect to the detecting plane 11A as shown by the broken rectangle showing the directions of the p-polarized light and s-polarized light at the detecting plane 11A. In this case, the larger the amount of the p-polarization component BEPn, the greater the amount of light leaking out to the projection lens 14 and the more a haze state is formed.
As one method to solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 6-175123 proposes the method of arranging the dichroic film inclined in the opposite direction with respect to the detecting plane 11A of the polarization beam splitter 11 and designing a dielectric multilayer film comprising the dichroic film so as to reduce the change in the state of polarization.
In this first method, the phase difference given to the p-polarization component and s-polarization component by the dichroic films at the boundary plane changes depending on the incident wavelength and angle of incidence. Therefore, it is possible to form a state sufficiently satisfying the change in the state of polarization for a specific wavelength and specific angle of incidence.
In the first method, however, it is difficult to obtain a satisfactory state for an incident wavelength and angle of incidence different from the specific wavelength and specific angle of incidence. In the final analysis, there are still problems in practice.
Further, Japanese Unexamined Patent Publication (Kokai) No. 10-26756, as shown in FIG. 4, proposes a second method of arranging the detecting plane 11A and dichroic films MB, MR in parallel and providing a quarter wavelength plate .lambda./4 in front of the reflection-type liquid crystal panel 13G so as to reduce the haze phenomenon.
In this case, among the direction cosines, only the direction cosines BC1 and BC6 differ due to the refraction of the polarization beam splitter 11, so BC2=BC3.noteq.BC1 and BC4=BC5.noteq.BC6. Further, the normal vectors become BD1.noteq.BD2.apprxeq.BD3 and BD4.apprxeq.BD5.apprxeq.BD6.
In this case, the relationship of the following formula is obtained from formulas (1) and (2) and the relationship between the direction cosines BCn and the normal vectors BDn. Note that the orthogonal p-polarization component BEPn becomes the same relationship. EQU BES1.apprxeq.BES2.apprxeq.BES3 (4)
As shown by the state of polarization in the case of application of the second method in FIGS. 5A to 5J from the comparison with FIGS. 3A to 3J, according to the second method, by arranging the detecting plane 11A and the dichroic films MB, MR, it is possible to make the p-polarization component and the s-polarization component substantially match (BES1.apprxeq.BES2.apprxeq.BES3) just before the detecting plane 11A (FIG. 5A), just before the dichroic film MB (FIG. 5B), and just before the dichroic film MR (FIG. 5D) and to reduce changes in the state of polarization.
Further, by arranging a quarter wavelength plate .lambda./4 with a retardation phase axis matched with the Y-axis, it is possible to make the optical image (FIG. 5F) emitted from the quarter wavelength plate .lambda./4 symmetrical with the Y-axis of the illumination light (FIG. 5E) incident on the quarter wavelength plate .lambda./4. Therefore, it is possible to make the p-polarization component and the s-polarization component substantially match Just before the dichroic film MR at the optical image (FIG. 5F), just before the dichroic film MB (FIG. 5H), and just before the detecting plane 11A (FIG. 5J) and possible to reduce the p-component (BEP6) incident on the detecting plane 11A.
However, considering the dependence of a quarter wavelength plate .lambda./4 on the angle of incidence and wavelength, when in the case of an index of refraction of the extraordinary ray Ne, the index of refraction of the ordinary ray No, and the thickness D, a phase difference is given to the planes of vibration by exactly a retardation shown in the following formula in the quarter wavelength plate .lambda./4, where, .DELTA.N=Ne-No, .lambda.=incident wavelength, and .theta.=angle of incidence: ##EQU1##
The quarter wavelength plate .lambda./4 is a phase difference plate with a .DELTA.ND of .nu.0/4 with respect to an angle of incidence .theta. of 0 and a specific wavelength of .nu.0. The phase difference given to the p-polarized light and the s-polarized light changes in accordance with the incident wavelength and angle of incidence on the path of the illumination light.
On this point, in the example explained in relation to FIGS. 5A to 5J, the phase differences given to the planes of vibration by the dichroic films MB, MR also change according to the incident wavelength and the angle of incidence. Due to this, depending on the quarter wavelength plate .lambda./4, when the directions of the p-polarization component and the s-polarization component differ even slightly, the light emitted from the detecting plane 11A due to linear polarization changes to elliptical polarized light depending on the angle of incidence and wavelength of the illumination light. In the end, at the stage where the optical image strikes the detecting plane 11A, it is no longer possible to sufficiently reduce the p-polarization component (BEP6) at the detecting plane 11A.
Further, even when the detecting plane 11A and dichroic films MB, MR are arranged in parallel, in practice the illumination light incident on and emitted from the polarization beam splitter 11 is light with a spread. The direction cosine changes due to the index of refraction of the polarization beam splitter 11 and the angle of incidence of the illumination light to the dichroic films MB, MR becomes larger.
Due to this, the illumination light shown in FIG. 5B incident on the dichroic film MB becomes elliptical polarized light. Further, when passing back and forth through the quarter phase plate .lambda./4, it is given a phase difference of at least 90 degrees according to the above formula (5). Due to this, in the illumination light, a state of non-symmetry about the Y-axis is formed between the case when emitted from the dichroic film MR toward the reflection-type liquid crystal panel 13G (FIG. 5E) and the case when reflected at the reflection-type liquid crystal panel 13G by non-polarization and striking the diohroic film MR.
In this case, when the optical image repeatedly passes through the dichroic films MR, MB and strikes the detecting plane 11A of the polarization beam splitter 11, it is difficult to make the p-component (BEP6) of the polarization beam splitter 11 the smallest and the p-component of the elliptical polarized light is emitted to the projection lens 14.
Due to these, it is difficult even with the second method to sufficiently reduce the haze phenomenon and increase the contrast of the display image.